The research proposed here employs single-unit recording of "head- direction" neurons in the anterior thalamic nuclei and the postsubiculum of freely moving rats. These cells selectively discharge when the rat's head is oriented in a particular direction, with respect to a landmark cue. It is believed that head direction cells play a fundamental role in spatial navigation, and the acquisition and maintenance of central representations of space. Spatial navigation is also influenced by internal or idiothetic cues (i.e. vestibular system). The experiments proposed here will examine the contribution of vestibular information to HD cells and spatial navigation using two approaches. First, the properties of HI) cells will be recorded in rats following acute or chronic disruption of the vestibular apparatus. HD cells will be recorded from vestibular-lesioned rats under conditions that require the use of idiothetic cues to guide spatial navigation. Secondly, the properties of HD cells will be evaluated following vestibular stimulation. These experiments will provide important neurophysiological determinants of the role of the vestibular system in supporting spatial cognition. Characterization of the vestibular influences upon HD cells will provide a further understanding of the neural bases of representations of space.